Method of forming closed sheet metal structures with an internal corrosion-resistant coating

ABSTRACT

This invention provides a method of forming partially or completely closed sheet metal structures having a corrosionresistant coating on the internal surface thereof by welding together shaped sheet metal parts to join the structure. In the method, the parts are formed from unshaped sheet metal stock having on at least one side thereof a coating of a coating material having a high content of one or more metals and/or graphite. The coating material is resistant to deep-drawing, is electrically conductive and is suitable for welding. The parts are formed in such a manner that the coated side will form the internal surface of the part in the assembled structure.

United States Patent 1191 Kuhnert 1 Oct. 1, 1974 I541 METHOD OF FORMING CLOSED SHEET 2,140,131 12/1958 Hclfrick 72/47 x METAL STRUCTURES WITH AN 3,153,990 10/1964 Kunzog 29/458 UX 3,252,215 5/1966 De Long et al 29/4511 INTERNAL CORROSION'RESISTANT 3,293,896 12/1966 Kompanek 29/527.2 x COATING 3,689,941 9/1972 Chartet 29/5272 x Inventor: Artur Kuhnert, Dachau, Germany Bayerische Motoren Werke AG, Munich, Germany Filed: Mar. 24, 1972 Appl. No.: 237,637

Assignee:

Foreign Application Priority Data Mar. 26, 1971 Germany 2114615 References Cited UNITED STATES PATENTS Primary ExaminerCharlie T. Moon Attorney, Agent, or Firm-Ladas, Parry, Von Gehr, Goldsmith & Deschamps [5 7 ABSTRACT This invention provides a method of forming partially or completely closed sheet metal structures having a corrosion-resistant coating on the internal surface thereof by welding together shaped sheet metal parts to join the structure. In the method, the parts are formed from unshaped sheet metal stock having on at least one side thereof a coating of a coating material having a high content of one or more metals and/or graphite. The coating material is resistant to deepdrawing, is electrically conductive and is suitable for welding. The parts are formed in such a manner that the coated side will form the internal surface of the part in the assembled structure.

10 Claims, 1 Drawing Figure METHOD OF FORMING CLOSED SHEET METAL STRUCTURES WITH AN INTERNAL CORROSION-RESISTANT COATING BACKGROUND OF THE INVENTION This invention is concerned with a method of forming partially or completely closed sheet metal structures having a corrosion-resistant coating on the internal surface thereof. Such structures are found, for example, in vehicle superstructures and floor groups, where the inner sides of the sheet metal parts are provided with a corrosion-resistant layer and the outer surfaces are provided with a multiple varnish coating, possibly with an additional corrosion-resistant covering or intermediate layer.

The protection from corrosion of completely or partially closed hollow spaces in vehicle superstructures or floor groups presents considerable difficulties, since the even application of a corrosion-resistant layer as well as the control of this coating are time-consuming and expensive. Coatings applied by dipping or electrophoresis after the assembly of the sheet metal parts have proved inadequate for the stresses caused by the salt-containing mist encountered in winter traffic, since the coating has insufficient strength and density. Anti-.

corrosive sprays can hardly be applied to all areas of the hollow spaces with the required eveness and with sufficient strength and density, even with high expenditure of time and labour.

The object of this invention is to provide a method of forming an even and dense corrosion-resistant layer on the internal surfaces of closed sheet metal structures by simple means and without substantial expenditure of time and labour.

SUMMARY According to the present invention, there is provided a method of forming partially or completely closed sheet metal structures having a corrosion-resistant coating on the internal surface thereof by welding together shaped sheet metal parts to form the structure, which comprises forming the parts from unshaped sheet metal stock having on at least one side thereof a coating of a coating material having a high content of at least one material selected from the group consisting of metals and graphite, said coating material being resistant to deep-drawing, electrically conductive and suitable for welding, said parts being formed such'that the coated side will form the internal surface thereof in the assembled structure.

BRIEF DESCRIPTION OF DRAWING The single FIGURE of the accompanying drawing illustrates a hollow frame for a vehicle floor group, the internal surfaces of which are provided with a corrosion-resistant layer according to the method of this invention.

DETAILED DESCRIPTION OF THE INVENTION With the method of this invention it IS'POSSibIG to provide in hollow spaces protection from corrosion which fully meets existing demands. Sheetmetal parts, the outer side of which forms a section of the exterior of the superstructure having a multiple layer coating, are expediently produced from sheet metal coated only on one side, whilst those which are so arranged that both sides are covered are preferably produced from sheet metal coated on both sides. The method according to the invention is very easy to carry out and requires no great expenditure, since the raw metal sheets are fully accessible and evenly coated. The coating may be carried out by means of various suitable processes, such as spraying on, pouring on, rolling on, dipping or by electrophoresis. If only one side is to be coated by dipping, the other side can be covered or pretreated in such a way that it is prevented from being coated. Pretreatments, for example, phosphatizing for improving the stability of the anticorrosive layer on the sheet metal parts, are easily carried out. As for timing, the coating can be carried out immediately before or after another operation, for example, immediately after the finishing pass in the mill before spooling the coils or before packing the flat sheets.

Coating materials which have the properties required and have proved particularly suitable are varnishes and paints based on synthetic resins with a content of metal powder, such as zinc powder and aluminium powder, and/or graphite powder. Besides the main properties for the specific application, i.e., the electric conductivity, in order to enable the metal sheets to be connected by welding and to permit further electrophoretic varnish coating, as well as the suitability for deep-drawing, in order that the coating adheres crack-free during forming, these materials are also stable to phosphatizing and washing solutions. Consequently, the conventional pretreatment of vehicle superstructures prior to the application of the usual varnish coat can be retained. If the corrosion-resistant layer is applied on one side and the coated side is used as the inner side, then the procedure for applying the paint or varnish coats to the visible outside of the superstructural parts is also not affected in any way.

The coating materials preferably contain at least 50 per cent by weight of metal and/or graphite powder. Particularly suitable coating materials comprise a binder mixture which can be cured by stoving and which consists of from 60 to per cent by weight of an epoxy resin containing free hydroxyl groups and having an OH number of from 0.35 to 0.50 mol OH/ g resin and of from 40 to 5 per cent by weight of polyfunctional compound which reacts with hydroxyl groups but does not react or virtually does not react with itself, such as poly-isocyanates, thioplasts of fully blocked aminoplasts; if desired, accelerators and solvents for the binder may be included.

In order that the invention may be more fully understood, the following Example is given by way of illustration only.

EXAMPLE A corrosion-resistant layer is rolled on to raw or pretreated deep-drawing sheet metal on one side during respooling of coils and stoved immediately afterwards at a temperature of about 200 to 250C. The coils ready for further processing by deep-drawing then have an uninterrupted unilateral corrosion-resistant layer extending over their whole length. The material for this layer is a zinc dust paint of the following composition: 51.0 parts by weight of zinc dust Z-620, spherical, degree of fineness 2-4p. zinc content; at least 95 percent of metalliczinc;

10.0 parts by weight of expoxy resin; 6.1 parts by weight of thioplast: a reaction product of dichlorodiethyl-formal with an alkali metal polysulphide; 0.2 parts by weight of accelerator: 2,4,6-tris-dimethy1- aminomethyl-phenol; 11.2 parts by weight of ethyl glycol acetate; 16.0 parts by weight of cyclohexanone; 5.0 parts by weight of xylene; and 0.5 parts by weight of Bentone 27.

This zinc dust paint is applied in a layer of from to 30p. thickness.

During deep-drawing of the metal sheets coated in this way, the coated side is so arranged that it forms that side of the sheet metal part, such as the outer part of a door threshold for a vehicle, which will subsequently be internal of the structure. When these sheet metal parts are assembled into vehicle superstructures or floor groups, the coatings of the coated inner sides of the sheet metal parts to be connected are in immediate contact. In this Example, the spot-welded flanges of the unilaterally coated outer part of the door threshold are in contact with spot-welded flanges of an inner part of the door threshold, which may be coated on both sides. These sheet metal parts are connected in the usual way by rolleror spot-welding, the flow of the current'through the electrically conductive coating remaining unimpaired. Due to the high melting temperature during the welding operation the coating likewise melts and permits immediate contact between the metal sheets, that is to say that the coating recedes in the immediate area of the welding process. When the welded areas cool down, the coating material reseals the metal surface so that the corrosion-resistant layer remains complete even in the welded areas.

When an electrophoretic primer dip-coating of the vehicle superstructure or floor groups is subsequently carried out, the electric conductivity of the corrosionresistant layer permits further coating of the internal surfaces of the structure with the aid of auxiliary electrodes arranged in the hollow spaces. The protection from corrosion is thus further increased. The outer sides of the sheet metal parts treated in this way are provided, irrespective of the properties of the corrosion-resistant layer applied to the inner sides, with several varnish coatings, a covering of varnish of undiminished high quality being thus achieved, possible with intermediate polishing operations.

The single FIGURE of the accompanying drawing illustrates a hollow frame for a vehicle floor group, the internal surfaces of which are provided with a corrosion-resistant layer according to the method of this invention.

As shown in the drawing, a hollow frame 1 of a vehiclc floor groups consists of two sheet metal parts 2 and 3 which are shaped like hat sections and are connected to each other with the aid of integral flanges 4 by spotweldcd areas 5. The sheet metal part 2 which is arranged in the interior of the vehicle, is coated on both sides with a corrosion-resistant layer 5 cosisting of a coating compound with a high metal content, which has been rolled on and stoved. The sheet metal part 3, on the other hand, which forms which a part 3 of its surface part of the skin of the vehicle, i.e., the door threshold provided with a door groove 3", is provided with the corrosion-resistant layer 6 only on its internal surface. Since the corrosion-resistant layer 6 extends to the adjacent connecting surface 4 of the flanges 4 and since the spot-welded areas 5 penetrate the corrosionresistant layer 6 only locally. the protection from corrosion in the hollow space 7 of the hollow frame 1 is uninterrupted. The surface of the sheet metal part 3 forming part of the skin with its part 3' can be provided with a multi-layer outer varnish coat after the whole floor group and the superstructure have been assembled. The electrically conductive corrosion-resistant layer 6 can also be provided with an additional coating by electrophoresis, the protection from corrosion being thus further increased.

I claim:

1. In a process for producing partially or completely closed sheet steel structures having a corrosionresistant coating on the inner surface thereof by first applying a coating which is electrically conductive, capable of being deep drawn without damaging the coating and suitable for welding on at least one surface of unshaped sheets of steel, deep drawing the coated sheets to form parts of the desired shape and welding the deep drawn coated parts together to form a sheet steel structure having a coated surface on the inside, the improvements wherein the coating is applied to the unshaped sheets of steel by means of a coating material having a high metal and/or graphite content and comprising metal powder and/or graphite powder and a hardenable bonding agent and the welding is carried out by electrical resistance welding.

2. A method according to claim 1 wherein the coating material is selected from the group consisting of synthetic resin paints and varnishes.

3. A method according to claim 2 wherein the coating material contains at least percent by weight of at least one material selected from the group comprising aluminium powder, zinc powder and graphite powder.

4. A method according to claim 2 wherein the sheet metal structure is subsequently subjected to an electrophoretic primer dip-coating.

5. A method according to claim 1 wherein the metal is zinc powder.

6. A method according to claim 1 wherein the metal is aluminium powder.

7. A method according to claim 1 wherein the hardenable bonding agent comprises from to percent by weight of an epoxy resin having free hydroxyl groups and having an OH number of from 0.35 to 0.50 mol OI-I/ g resin and of from 40 to 5 per cent by weight of polyfunctional compound selected from the group comprising poly-isocyanates,' thioplasts and fully blocked aminoplasts.

8. In a process for producing partially or completely clolsed sheet steel structures having a corrosionresistant coating on the inner surface thereof by first applying a coating which is electrically conductive, capable of resisting high forming stresses and is suitable for welding, on at least one surface of unshaped sheets of steel deforming the sheets steel parts of the desired shape and welding the deformed parts together to form a steel metal structure having a coated surface on the 7 inside, the improvements wherein the coating is applied 3,838,495 6 9. A method according to claim 8 wherein the coated ing material contains at least 50 per cent by weight of side has been phosphatized prior to application of the at least one material selected from the group consisting coating. of metals and graphite.

10. A method according to claim 8 wherein the coat- 

1. IN A PROCESS FOR PRODUCING PARTIALLY OR COMPLETELY CLOSED SHEET STEEL STRUCTURES HAVING A CORRISION-RESISTANT COATING ON THE INNER SURFACE THEREOF BY A FIRST APPLYING A COATING WHICH IS ELECTRICALLY CONDUCTIVE, CAPABLE OF BEING DEEP DRAWN WITHOUT DAMAGING THE COATING AND SUITABLE FOR WELDING ON AT LEAST ONE SURFACE OF UNSHAPED SHEETS OF STEEL,DEEP DRAWING THE COATED SHEETS TO FORM PAETS OF THE DESIRED SHAPE AND WELDING THE DEEP DRAWN COATED PARTS TOGETHER TO FORM A SHEET STEEL STRUCTURE
 2. A method according to claim 1 wherein the coating material is selected from the group consisting of synthetic resin paints and varnishes.
 3. A method according to claim 2 wherein the coating material contains at least 50 percent by weight of at least one material selected from the group comprising aluminium powder, zinc powder and graphite powder.
 4. A method according to claim 2 wherein the sheet metal structure is subsequently subjected to an electrophoretic primer dip-coating.
 5. A method according to claim 1 wherein the metal is zinc powder.
 6. A method according to claim 1 wherein the metal is aluminium powder.
 7. A method according to claim 1 wherein the hardenable bonding agent comprises from 60 to 95 percent by weight of an epoxy resin having free hydroxyl groups and having an OH number of from 0.35 to 0.50 mol OH/100 g resin and of from 40 to 5 per cent by weight of polyfunctional compound selected from the group comprising poly-isocyanates, thioplasts and fully blocked aminoplasts.
 8. In a process for producing partially or completely clolsed sheet steel structures having a corrosion-resistant coating on the inner surface thereof by first applying a coating which is electrically conductive, capable of resisting high forming stresses and is suitable for welding, on at least one surface of unshaped sheets of steel deforming the sheets steel parts of the desired shape and welding the deformed parts together to form a steel metal structure having a coated surface on the inside, the improvements wherein the coating is applied to the unshaped sheets of steel by means of a coating material having a high metal and/or graphite content and comprising metal powder and/or graphite powder and a synthetic resin hardenable bonding agent and the welding is accomplished by electrical resistance welding.
 9. A method according to claim 8 wherein the coated side has been phosphatized prior to application of the coating.
 10. A method according to claim 8 wherein the coating material contains at least 50 per cent by weight of at least one material selected from the group consisting of metals and graphite. 